1. Field of the Invention
The present invention relates generally to side-group liquid-crystalline polymers which, among other things are utilized as optical data storage media. More particularly, the present invention relates to methods for aligning the nematic phase of such liquid crystalline polymers in order to provide for erasure of stored optical data.
2. Description of Related Art
The publications and other reference materials referred to herein to describe the background of the invention and to provide additional details regarding its practice are hereby incorporated by reference. For convenience, the reference materials are numerically referenced and identified in the appended bibliography.
Liquid-crystalline polymers (LCPs) offer a wide range of valuable material properties (1). The most familiar example is the outstanding tensile strength of fibers and films manufactured from main-chain liquid-crystalline polymer solutions or melts. Their strength arises from the enhanced molecular orientation that is achieved when extensional deformation is applied to a fluid with local orientational order.
Another type of liquid-crystalline polymer has the mesogenic group pendant to the polymer backbone as a side-group or side-chain. The side-group LCPs have been developed for potential optical applications as holographic information storage media or nonlinear optical materials (2).
For optical data storage, a plate formed from the side-group LCP is annealed in an orienting field to yield a sample that has uniform molecular orientation or alignment. The resulting "monodomain" material provides a transparent, highly birefringent background onto which opaque elements can be written by local heating with laser light. The laser is used to heat a spot as small as 10.mu. in diameter through the nematic to isotropic transition. When the spot cools in the absence of an applied field, it leaves a region (pixel) that is riddled with orientation defects. This island of "polydomain" material strongly scatters light, and is consequently opaque. Both the monodomain and polydomain states can be frozen-in by cooling through the glass transition of the polymer.
The present established method for erasing the opaque pixels once they are written is slow and cumbersome. The method involves producing uniform macroscopic orientation by prolonged annealing in a high electric or magnetic field or by contact with an orienting surface. As a result of the slow erasure procedure, side-group LCP materials have been viewed primarily as long term (write once read many times, or WORM) storage media. In order to expand the use of side-group LCPs into read-write-erase data-storage media, a convenient, quick and simple method must be provided which is capable of providing uniform orientation or alignment of the side-group LCP. In addition, if a convenient method for erasing opaque data regions can be developed, the use of side-group LCPs may also expand into thermal and optical sensors.
Considerable work has been conducted to investigate the rheology of main chain (3) and rigid-rod (4) polymer liquid crystals. However, very few studies have been made that investigate the rheology of side-group liquid crystalline polymers. Accordingly, no methods have been developed to erase the opaque pixels in side-group LCPs using flow. Of the few studies which have been conducted on the flow behavior side-group liquid crystalline polymers, one study concluded that the use of shear or shear flow could not be used to enhance the orientational order or align side-group liquid crystalline polymers (5).
As is apparent from the above, there is a continuing need to investigate and develop new methods for aligning side-group liquid crystalline polymers so that their use in potential optical applications as data storage media and thermal and optical sensors can be expanded.